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EXPLANATION OF THE COLOR CHWART FOR THE DESCRIPTION OF 
SEDIMENTARY ROCKS, PREPARED UNDER THE AUSPICES OF 


THE DIVISION OF GEOLOGY AND GEOGRAPHY, OF THE NATIONAL 
RESEARCH COUNCIL 


By Marcus I. GotpMAN and H. BE. MERwIN 


In preparing this chart colors were selected from Ridgway* with the 
aim to cover systematically the range of colors of rocks. These were arranged 
according to Ridgway’s system and reproduced as accurately as possible 
lithographically. 


In Ridgway’s system the numbers at the heads of the columns desig- 
nate the hues. The hue is the position of a color in the spectrum. Ridgway 
has selected from the spectrum 59 hues which he has named and numbered 
consecutively from 1 red to 59 violet. Thus, 11 is orange, 23 yellow, 35 green, 
and 49 blue. Intermediate hues are named by a combination of the names 
of the hues between which they lie, as for instance yellow-orange, orange- 
yellow, and so on, the last term of the combination being the name of the 
dominant hue. He arranged the hues in horizontal succession across the 
middle of the page. Upward from the middle, he made his colors lighter by 
adding white, designating the increasing whiteness by letters running from 
a tog. Downward he darkened his colors, designating the increasing dark- 
ening by letters running from h to n. In addition to this set of modified pure 
hues there are five sets of hues each made grayer than the preceding by the 
addition of increasing amounts of an intermediate neutral gray about like. 
next to the last neutral gray on the accompanying chart (Plate 2). The 
progressive graying is designated in Ridgway’s book by prime signs running 
from one to five, thus (’—”’’’’ ). Each hue thus grayed is then lightened and 
darkened like the pure hues. In addition to these sets of colors he has a 
column of neutral grays which are graduations between black and white. 


In selecting and arranging for this chart the colors from Ridgway, the 
plan was to reproduce entire columns and to arrange them in order of hues 
and under the hues in order of grayness from left to right. Technical diffi- 
culties in reproducing the colors, however, have made it necessary to elimi- 
nate some of the colorst and to depart somewhat from the regularity of 
Ridgway’s arrangement. The principles of his arrangement have, however, 
been retained. Two columns which fell out of place have been carried over 
with arrows to their proper position. In designating grayness, numerical 
exponents have been used instead of prime signs. The colors that have been 
the starting points for lightening upward and darkening downward, to 
which Ridgway gave no letter, have on this chart been designated by the 
letter 0, 


* Ridgway, Robert, Color standards and color nomenclature, Washington, D. C., pub- 
lished by the author, 1912. 
t The eliminated color squares have been printed over with aluminum. 


The approximate Ridgway equivalent is given in this modified form at 
the left of each color square, with the letter at the bottom, the hue number 
above it, and the exponent designating stage of grayness above that. Perfect 
agreement between these colors and Ridgway’s is not to be expected, because 
some of Ridgway’s colors having the same designation are appreciably 
different in different copies of his book, and the colors of the accompanying 
chart are slightly different on different sheets. But any of these differences 


are less than the observational errors in comparing rocks with the printed 
colors.. 


A sample of rock may not match any of these colors. But from a group 
of colors which it most nearly matches an approximate interpolation can be 
made and designated by intermediate symbols. Thus a color about midway 
between 6 m and 82k is 741. Or, to take a more complicated example: A rock 
more yellow than 18 but not quite as yellow as 21, grayer than 21? but not 
as gray as 21°, and between o and J in darkness, might be 2071/*h; or a rock 


between 1 and 8 in hue, and nearer 8, about as gray as 1° or 13°, and as dark 
as 7k or 871, might be 5°17. 


The designation by symbols is fundamental to the use of this chart. 
The symbol should always be given. Desirable as it is to convey the con- 
ception of a color to another person in some simple way, it seems that in 
the absence of generally disseminated and generally accepted standards 
this is not possible. Aside from the training it affords in the understanding 
of color, this chart will be of use mainly to the geologist himself and as a 
means of communication between those who use it either by itself or in 
conjunction with Ridgway’s book. The designation by the symbols of this 
System may be supplemented by verbal description, in which we believe the 
most fundamental term is the designation by hue. Thus “a medium gray 
dark reddish orange” (equal, for example, to 8*J) or “a slightly gray light 
-yellow”’ (equal, for example, to 211d). 


Certain popular terms, such as chocolate (Ridgway’s 7m), olive (212m), 
buff, (18°d), maroon, (1°m) may also be useful to supplement the designa- 
tion by Symbols. But the indefiniteness of most of them, unless used in a 
definite scheme such as Ridgway’s, should be realized. According to 
Ridgway’s investigations, “drab,” one of the very commonly used terms of 


this kind, is applied to a great range of colors clustering around 17‘ (near 
18*a of the accompanying chart). 


i may be worth while to point out that browns are dark orange colors, 
than olive” is a dark yellow, and that the grayish reds of the type of 1%a to i, 
1**/¢c to j are commonly thought of as purplish but are best not so described. 


In matching it is desirable to hold a small fragment of the rock close to 
the colors and to view them by diffused light coming from behind. 


We recommend that in the office, as far as possible, the use of this chart 
be supplemented by the use of Ridgway’s book, which may be had from 
A. Hoen & Company, Chester, Chase, and Biddle Streets, Baltimore, Maryland, 


for plus postage (shipping weight 2 pounds). The book has 1115 colors; 
this chart has only 114, 


Marcus I. GotpMAN, Chairman, 


H. E. MERwIN, 


Subcommittee on Color Chart of the Committee on 
Sedimentation of the Division of Geology and 
Geography, National Research Council. 


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COLOR CHART FOR FIELD DESCRIPTION OF SEDIMENTARY ROCKS | 
NATIONAL RESEARCH COUNCIL, WASHINGTON, D. C. 


Prepared under the auspices of the Division of Geology and Geography by 
Marcus |. Goldman and H. E. Merwin, 1928 


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COLOR CHART FOR FIELD DESCRIPTION OF SEDIMENTARY ROCKS 
NATIONAL RESEARCH COUNCIL, WASHINGTON, D. C. 


Prepared under the auspices of the Division of Geology and Geography by 
Marcus |. Goldman and H. E. Merwin, 1928 


